This invention relates to signal-to-noise ratio improvement in magnetic resonance apparatus, such as magnetic resonance imaging apparatus or magnetic resonance spectroscopy apparatus.
A typical magnetic resonance imaging apparatus is shown in FIG. 1. A patient 1 on a couch 2 is slid into the bore 3 of an annular electromagnet, typically a superconducting electromagnet. A main magnetic field is generated in alignment with the axis of the bore, and gradient coils (not shown) are provided to set up magnetic field gradients for example along the z-direction along the axis of the bore, and along x and y directions in the radial plane. A transmit coil 4 surrounds the patient and transmits pulses of r.f. energy to excite to resonance magnetic resonance active nuclei such as protons in the region of the patient to be examined. This transmit coil 4 is normally surrounded by an r.f. shield coil 5 to shield the bore 3 of the electromagnet from extraneous unwanted r.f. signals. The transmit coil 4 can be also be used to receive the magnetic resonance signals which result from the resonating protons in the region of interest, although a separate receive coil is often provided. For many examinations, a coil placed on the surface of the patient is used to receive the magnetic resonance signals, such as the coil 6 (shown on an enlarged scale in FIG. 2).
The signal picked up by the receive coil 6 is of course tiny. It might be necessary to collect data from a region of the body several times, with a consequent increase in the scanning time, in order to achieve satisfactory signal-to-noise ratio in the resulting image.
There are various sources of noise which accompany the desired magnetic resonance signal in the signal picked up by the receive coil 6.
In all except the lowest values of main magnetic field, the dominant noise is actually derived from the body rather than from the coil. Much of that noise comes from regions closest to the coil windings, which contribute a disproportionate amount relative to the value of the data obtained from them. The body has electrical resistances and associated with them noise voltages, and thus currents which generate the noise signals detected by the coil.
If the region it is desired to image is at some depth in the patient, as is often the case, (for example the region 7), the coil 6 is normally pressed as close to the patient's skin as practicable, because the strength of the signal from the region 7 is (simplistically) inversely proportional to the distance of the region 7 from the coil 6 (the signal strength being dependent on the so-called filling factor).
The disadvantage of this is an increase in noise from the regions of the body adjacent to the coil.
While the individual thermal sources of noise are of low intensity, there are many of them in the vicinity of the coil. For example, flux 9 (shown dotted) from one particular noise source 8 couples with the receive coil 6 (FIG. 2). There would be many such noise sources 8. Stronger flux 10, 11 emanating from a signal source 12 in the region of interest 7, would also couple with the receive coil 6.
The noise would be greatly reduced if the coil 6 was stood off from the surface of the patient such as to the position shown dotted, but the signal would also fall off to the extent that the signal-to-noise ratio would in fact be worse. To take the signal source 12 as an example, some of the flux such as 10 would still couple with the plane of the receive coil 5, but other flux 11 would not.
Another source of noise in the signal picked up by the coil 6 stems from the coil windings themselves, and this can be reduced by refrigerating the coils or by using superconducting coils. Because of the refrigeration, substantial thermal insulation is needed between them and the patient so removing them from the surface of the patient and, while the noise from the body is also thus reduced, there is still an overall loss of signal-to-noise ratio because of the loss of filling factor.
The applicants are aware that microstructures comprising an array of capacitive elements made from non-magnetic conducting material can exhibit magnetic permeability at radio frequencies (IEEE Transactions on Microwave Theory and Techniques, Vol. 47, No. 11, November 1999, Magnetism from Conductors and Enhanced Non-Linear Phenomena by J B Pendry, A J Holden, D J Robbins and W J Stewart, and International Patent Application WO 00/41270).